The Devil's Slide Tunnels are the latest replacement for the original Half Moon Bay-Colma Road (HMB-CR), which opened in 1879 and remained in use until it was replaced by the Old Pedro Mountain Road in 1915. The current trail follows the old road bed as closely as possible. After automobiles started using it, historic sources describe a treacherous narrow road with steep, rutted hairpin turns that led to many accidents. The steepest sections must have been near the top of the ridge, where I can find no obvious evidence of the road.
I've only seen this trail mapped on bicycle sites where it is listed as Old Colma Bicycle Route. The southern side is mostly maintained by mountain bikers. On the southern side, you need to be careful to avoid poison oak in a few places.
The start is about 0.15 miles from the northern parking lot of the Devil's Slide Trail
. A path, intentionally too rough for motorcycles, goes around the black gate that blocks the dirt road going up the hill. Behind the gate, the road climbs steeply for about 150 yards until it intersects the HMB-CR coming up from the private Shamrock (horse) Ranch.
Although the road-bed is wide enough for a car, it is often overgrown so that there is only a narrow path between flowers and bushes that are sometimes more than head high. The road winds gradually up for about 0.7 miles and then climbs more steeply directly to the top of the ridge. Just before the steep section, there is a brushy trail of use that leads to a point overlooking the Devil's Slide Trail
, the Headlands, and San Pedro Rock
At the top of the ridge, the branch trail to the left follows the ridge until it intersects with Old San Pedro Mountain Road
For the HMB-CR, make a half right turn to a narrow trail that descends somewhat steeply down the south side of the mountain to a small rocky point. From here, there are sweeping views of the coastline and Montara. From this point on, the old roadbed is obvious. As the trail swings around the point and descends towards a small creek, you can see a mountain bike trail that winds back up to the top of the ridge.
HMB-CR cuts back towards the coast, passing above the high rocky promontory called the Second Lookout, and then makes a sharp switchback A little beyond the switchback, you can see where the HMB-CR was cut into the rocky cliffs above HW1. As you run this, you have great views of the rocky promontories and coastline below. When the roadbed disappears, return the way you came.
Trail is densely surrounded by more than 70 different species of flowering plants and bushes. The flowers and greenery remain here later than most other areas.